The most influential Catholic in England and Wales has warned that same-sex marriages would be a "profoundly radical step" that would threaten the true meaning of matrimony, it has been reported.

Plans to extend marriage to same-sex couples would strip the union of its "distinctive nature" and reduce it to just the commitment of the two people involved, without recognising the intention of procreation and the education of children, the Archbishop of Westminster, the Most Reverend Vincent Nichols, will say.

The warning is part of a letter seen by the Daily Telegraph which is due to be read from the pulpit in 2,500 churches during Mass this Sunday.

The letter is co-signed by the Archbishop of Southwark, the Most Reverend Peter Smith, and comes as ministers prepare to launch a consultation on the introduction of same-sex marriages.

The archbishops argue that marriage is a "natural institution" with a meaning understood by wider society outside the Church. The say that extending it to same-sex couples would reduce its significance.

In the letter, the archbishops say: "Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now. The law helps to shape and form social and cultural values.

"A change in the law would gradually and inevitably transform society's understanding of the purpose of marriage. It would reduce it just to the commitment of the two people involved. There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children.

"We have a duty to married people today, and to those who come after us, to do all we can to ensure that the true meaning of marriage is not lost for future generations."

The letter says the roots of marriage lie in human nature and the pattern of complementarity and fertility in the union are affirmed by many other religious traditions.

It argues that matrimony is an expression of "our fundamental humanity" and says: "Neither the Church nor the State has the power to change this fundamental understanding of marriage itself."

It continues: "Its status in law is the prudent fruit of experience, for the good of the spouses and the good of the family. In this way society esteems the married couple as the source and guardians of the next generation. As an institution marriage is at the foundation of our society."

The archbishops say they recognise the experience of those who have suffered the breakdown of a marriage or a bereavement, adding: "Many provide a remarkable example of courage and fidelity. Many strive to make the best out of difficult and complex situations. We hope that they are always welcomed and helped to feel valued members of our parish communities."

The letter argues that the current law does not unfairly discriminate against same-sex couples but instead preserves the crucial components of the union.

"The reasons given by our government for wanting to change the definition of marriage are those of equality and discrimination. But our present law does not discriminate unjustly when it requires both a man and a woman for marriage. It simply recognises and protects the distinctive nature of marriage.

The letter has been sent to bishops across England and Wales and is being circulated to parishes this week, the Daily Telegraph reported.

It is accompanied by a cover note asking priests to encourage their parishioners to sign a petition set up by Lord Carey's Coalition For Marriage, opposing the redefinition of marriage, the paper said.

The head of the Catholic Church in Scotland earlier described marriage between couples of the same sex as an "aberration" which would lead society even further into "immorality".

Cardinal Keith O'Brien told BBC Radio 4's Today programme it would be the "thin end of the wedge", comparing it to the legalisation of abortion in 1967.

"Further aberrations would be taking place and society would be degenerating even further than it has into immorality," he said.

"It is changing the whole notion of what marriage and what a family is. Children have the right to have a mother and a father."

The Cardinal said change would take the accepted definition of marriage as a relationship between a man and a woman and turn it on its head.

David Cameron's official spokesman responded: "The government has made clear its commitment to equality. We believe people should have the option of civil marriage, irrespective of sexual orientation."

He also said that the timing of the consultation had been chosen to allow legislation to be taken through parliament before the general election planned for 2015.

But he declined to speculate on the timetable for any bill, telling reporters: "We normally set out the timetable for legislation at the time of the Queen's Speech and we do it session by session.

"We have purposefully set up this consultation with a timetable that would allow us to have that legislation in this parliament."

Nick Clegg told ITV's The Agenda: "I think marriage is defined surely by two people who love each other and want to make a commitment to each other.

"Depriving same-sex couples of that public display … in this day and age I think most people think 'come on, let's move with the times'."

Equalities minister Lynne Featherstone previously said: "This government is promoting a fair society where people respect each other.

"I believe that if a couple love each other and want to commit to a life together, they should have the option of a civil marriage, irrespective of whether they are gay or straight.

"We are absolutely not changing religious marriage, or requiring religious groups to go against their traditions.

"We will be working closely with all those who have an interest in the area to understand their views ahead of the formal consultation in March."